This proposal is intended to provide additional training in child development, demography, and advanced statistical methods and to support research examining relationships between family structure transitions, parenting behaviors, and children's cognitive and behavioral development. A considerable body of research has been devoted to marital dissolution and associations between single-parent family structure and child wellbeing. In contrast, the effects of maternal re-partnering on children have received less attention, despite estimates that nearly a third of children will spend some time in stepfamilies. This research will focus on the effects of maternal re-partnering on parenting behaviors and child development, paying specific attention to whether there are differences in these effects when such re-partnerings constitute cohabitations, as opposed to marriages. It has 4 specific aims: (1) To estimate the effects of changes in family structure, with an emphasis on maternal re-partnering, on changes in children's cognitive and behavioral development; (2) To explore the effects of maternal re-partnering on changes in maternal parenting behaviors; (3) To compare the types of parenting behaviors and discipline strategies utilized by biological fathers to those utilized by socialfathers (i.e., men who are married to, or cohabiting with, women and their biological children, but who are not those children's biological fathers); and (4) To examine whether and how changes in parenting behaviors may explain (i.e., mediate) associations between changes in family structure and changes in children's achievement and behavior. These aims will be addressed using data from 3 population based longitudinal studies: the Fragile Families and Child Wellbeing Study, the Panel Study of Income Dynamics, and the National Longitudinal Survey of Youth. All analyses will specificallytest whether there are differences in associations between maternal re-partnering and parenting behaviors and child outcomes by marital status. They will also consider whether the impact of maternal re-partnering varies by child age and gender, as well as family socioeconomic status. Finally, they will assesswhether effects of maternal re-partnering on parenting and child wellbeing are transitory or persist over time. As a sizeable proportion of children will experience maternal re-partnershipsduring the course of their childhoods, it is crucial to understand the changes in parenting behaviors, family dynamics, and child development that may accompany these transitions. This research has implications for public policies intended to encourage marriage and family formation, as well as for designing programs and policies to promote child wellbeing for children in complex family structures.